538 



MUSCULAR SYSTEM. 



other in a longitudinal series, and generally 

 furnished with lateral appendages of different 

 kinds symmetrically disposed, which are sub- 

 servient to many and very various purposes. 

 The nervous system, moreover, throughout the 

 entire range of this extensive series assumes a 

 new and constant arrangement in itself, quite 

 sufficient to characterize this sub-kingdom of 

 animated nature, and with the different modifi- 

 cations of this portion of their economy are 

 intimately connected the progressive changes 

 observable in the structure and habits of the 

 different classes included therein. 



In the simplest conceivable condition under 

 which a Homogangliate animal could exist, and 

 doubtless among the lowest of the red-blooded 

 worms and most imperfect forms of insect 

 larvce such a condition might be pointed out, 

 the body would consist of a long succession of 

 similar rings, each of which would contain an 

 appropriate nervous apparatus consisting of a 

 pair of ganglia symmetrically disposed on each 

 side of the mesian line, from which nerves 

 proceed for the innervation of the segment in 

 which the brains or ganglia were placed. These 

 ganglia in each segment communicate with each 

 other and likewise with the pairs that precede 

 and follow them by inter-communicating ner- 

 vous filaments, and thus the entire series of 

 individual brains or ganglia is united into one 

 system made up of as many pairs as there 

 would be rings entering into the composition 

 of the body. There is, however, a remarkable 

 difference between the anterior pair of ganglia 

 and those placed in the succeeding segments. 

 The first pair is invariably situated above the 

 oesophagus on its dorsal aspect, while all the 

 rest are arranged beneath the alimentary canal 

 along the ventral region of the animal, so that 

 the nervous cords that join the first and second 

 pairs of ganglia embrace the cesophageal tube. 

 The supra-cssophageal pair of brains invariably 

 communicates with the instruments of the 

 senses whenever such exist, and therefore is 

 very justly comparable to the encep/ialon of 

 higher animals; while the succeeding chain of 

 sub-02sophageal ganglia animate the muscles of 

 the different segments of the body, and may 

 therefore be looked upon with great reason as 

 representing the spinal cord of the Vertebrata. 



But while the ganglia either of the head or 

 of the ventral cord are thus numerous, as we 

 have supposed them to be, in the lowest worm, 

 they are as> yet by far too small and devoid of 

 energy in such a dispersed condition to corres- 

 pond with organs connected with the higher 

 senses, or even to wield muscles of sufficient 

 power to support the weight of the body raised 

 on articulated limbs. Therefore, before senses 

 can be given or active limbs bestowed, a pro- 

 cess of concentration must be gone through, the 

 encephalic masses must be enlarged and thus 

 rendered more perfect, the gangl ionic centres 

 that influence muscular movements reduced in 

 number and made proportionately more ener- 

 getic, and exactly in the ratio in which this 

 improvement is effected in the nervous system, 

 do the muscles become by degrees stronger and 

 more efficient, and the limbs appended to the 



body more active and useful as locomotive 

 agents. This, however, will be best exempli- 

 fied by a rapid survey of the principal classes 

 that compose the division of animals we are 

 now considering. 



In the lowest Annelidans, as for example the 

 Gordtns or hair-worm, so impotent are the 

 minute ganglia bestowed, that even the rings 

 upon the exterior of the body are scarcely 

 indicated, and not the least vestiges either of 

 limbs, tentacula, or eyes are to be detected. 

 In the Leeches even, although the number of 

 ganglia is in them considerably diminished, and 

 rings of the body more strongly marked, exter- 

 nal limbs cannot as yet be given, their place 

 being supplied by the suctorial discs of the 

 head and tail; nevertheless, even in these 

 aquatic Annelides, the encephalic masses are 

 sufficiently advanced to permit organs of vision 

 to be granted, and accordingly for the first time 

 in the animal series (as far as our own belief 

 goes) are real eyes met with. The muscular 

 system in these humble worms consists exclu- 

 sively of the contractile walls of the body, the 

 fibres of which are arranged in three strata, 

 superposed one upon the other, and pass in 

 different directions, one stratum being com- 

 posed of longitudinal, another of obliquely 

 spiral, and another of annular fibres surround- 

 ing the body of the animal; but these are 

 sufficient for progression, all the movements of 

 contraction, elongation, or flexure of the body 

 being provided for by this simple arrangement. 



In the Nereis, Aphrodite, and other erratic 

 worms, external appendages become developed 

 from the lateral aspects of the different seg- 

 ments, in the shape of bundles of setae, moved 

 by muscles appropriated to each set, and these 

 constitute the first rudiments of locomotive 

 limbs. The senses are at the same time im- 

 proved in their condition, tentacula or feelers 

 are found appended to the head, and the eyes 

 become larger and more conspicuous, although 

 still presenting the form of simple specks or 

 ocelli. 



In the MYRIAPODA the limbs become arti- 

 culated, and of sufficient strength to permit of 

 a terrestrial existence, each one of the numerous 

 legs having a distinct set of muscles appro- 

 priated to its movements, in addition to the 

 muscular apparatus, whereby the segments of 

 the as yet flexible and elongated body are 

 endowed with motion, and which of course 

 represent the strata of the muscular covering of 

 the Leech, strengthened and endowed with the 

 capability of more precise action, in proportion 

 as the cuticular skeleton has become more 

 dense and distinctly jointed. Antennae are, 

 moreover, now placed upon the head resem- 

 bling those of Insects, no doubt constituting 

 organs of sense of a similar nature, and the 

 eyes in Scolopendra are found to be very dis- 

 tinct and perfectly organized, but still only 

 ocelli resembling the simple eyes of Insects. 



In the admirably constructed class of IN- 

 SECTS, creatures adapted to an aerial existence, 

 and consequently requiring the utmost exertion 

 of muscular power to sustain their bodies in 

 the air, the muscular system of the locomotive 



